[Bug c/102909] New: Missing -Wunused-but-set-variable warning

[Bug c/102909] New: Missing -Wunused-but-set-variable warning

[mytbk920423 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102909

            Bug ID: 102909
           Summary: Missing -Wunused-but-set-variable warning
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mytbk920423 at gmail dot com
  Target Milestone: ---

GCC misses the -Wunused-but-set-variable in the some code (GCC and Clang output
can be seen in https://godbolt.org/z/7658M4qra).

I first found a bug caused by the following code, in which no compilers warn:

void ioapic_set_max_vectors(void *ioapic_base, int mre_count)
{
        u32 reg;
        u8 count;
        reg = io_apic_read(ioapic_base, 0x01);
        count = reg >> 16;
        if (mre_count > 0)
                count = mre_count - 1;

        // reg is defined but not used after this
        reg &= ~(0xff << 16);
        reg |= count << 16;
        // ``count`` should be ``reg`` in the following line
        io_apic_write(ioapic_base, 0x01, count);
}

I think that's because the variable ``reg`` is only unused in part of the
control flow, so I change the code as following. However, GCC doesn't warn on
this either.

void ioapic_set_max_vectors(void *ioapic_base, int mre_count)
{
        u32 reg;
        u32 new_reg;
        u8 count;
        reg = io_apic_read(ioapic_base, 0x01);
        count = reg >> 16;
        if (mre_count > 0)
                count = mre_count - 1;

        // new_reg is defined but not used
        new_reg = reg & (~(0xff << 16));
        new_reg |= count << 16;
        // ``count`` should be ``new_reg`` in the following
        io_apic_write(ioapic_base, 0x01, count);
}

[Bug c/102909] Missing -Wunused-but-set-variable warning

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102909

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
The warning works as documented.  It warns about variables that are only stored
to and never read.  new_reg in the second testcase or reg is not such a
variable, it is also read.

[Bug c/102909] Missing -Wunused-but-set-variable warning

[mytbk920423 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102909

--- Comment #2 from Iru Cai <mytbk920423 at gmail dot com> ---
So it looks something like https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44677

GCC thinks ``a`` is set but not used in ``a = 1 + b;``, but is used in ``a = 1;
a += b;``.

[Bug c/102909] Missing -Wunused-but-set-variable warning

[mytbk920423 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102909

--- Comment #3 from Iru Cai <mytbk920423 at gmail dot com> ---
Looks like this kind of things are detected in the front-end. The GNAT
front-end can warn on the similar things:

procedure Main is
        A : Integer;
        B : constant Integer := 1;
begin
        A := 0;
        A := A + B;
end Main;

$ gcc -O2 -gnatwa -gnatwe -c main.adb 
main.adb:6:09: warning: possibly useless assignment to "A", value might not be
referenced

[Bug c/102909] Missing -Wunused-but-set-variable warning

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102909

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 44677.

*** This bug has been marked as a duplicate of bug 44677 ***